Pattern definition for the production of semiconductor devices and photomasks is accomplished through the use of materials called resists. Resists which are typically organic polymers undergo a change is solubility response to a developer solution upon exposure to an irradiation source such as UV-visibile light, x-rays or a beam of charged particles. By definition, if the exposed areas attain a higher dissolution rate relative to the unexposed areas, the resist is referred to as positive acting.
Most positive resists are produced from the combination of three basic ingredients: a resin, a photosensitive compound (sensitizer) and the solvent system. It is the role of the sensitizer to provide the resist its ability to withstand dissolution in the developer system. However, upon exposure to an irradiation source, the sensitizer is chemically transformed to a developer-soluble material. The net effect is that the dissolution rate of the exposed areas is greatly increased. However, the difference in dissolution rates between exposed and unexposed domains of the resist is only relative i.e. the unexposed resist may dissolve to some extent in the developer. For practical working resists, the differential solubility should be a factor of about 30 or greater i.e. film loss from unexposed resist should be less than 3%. The change in the differential solubility is not, however, a continuously linear function of exposure dose. Rather, at low exposures the dissolution rates between exposed and unexposed areas of a resist are not too dissimilar. As the dose is increased, a threshold value is reached where the dissolution rate increases sharply. Optimum exposure corresponds to the minimum dose which allows complete removal of exposed resist for a predetermined set of processing conditions. To quantify differences among resists, the term sensitivity may be defined as the minimum radiation dose which provides the desired ratio for the rate of dissolution between exposed and unexposed portions of the resist for a given set of parameters.
In the fabrication of semiconductor devices and photomasks, it is advantageous to use a resist of highest sensitivity (minimum exposure time) in order to maximize throughput. This premise applies to any lithographic procedure irrespective of the irradiation source but is particularly important for direct write operations utilizing charged particles (e.g. electrons and ions). Currently, the chief impediment to the employment of these techniques as routine processing operations is their limited throughput. This limitation is especially severe for electron beam lithography which is cost prohibitive compared to the more commonly used UV-visible photolithography. Exceptions are those special applications which require geometries not attainable by the optical techniques. Moreover, it is anticipated that future need for smaller geometries will not be within the capabilities of photolithography to faithfully print the required details. Hence, electron and ion beam lithography are attractive alternatives to meet future needs. This realization has stimulated much research into these lithographic methods with particular emphasis on improving the sensitivity of existing resists and/or development of new resist systems.
Accordingly, a need is present for a method which would obviate the disadvantage of low throughput due to limited sensitivity. Attempts to improve the sensitivity of electron beam resists have focused on the development of novel systems. Pre-exposure of a commercial photoresist with monochromatic light from an argon laser is reported for the recording of holograms as described by Beesley and Castledine, Applied Optics, vol. 9, 1970, pp. 2720-2724. However, values relating to film loss from unexposed resists are not known to have been considered in that reported study. In any event, it has not been known prior to any discovery that pre-exposure may be usefully applied to electron beam or optical developing techniques in the manner and for the particular purpose presented by this invention.